This invention relates to polycarbonate, and the production thereof, where a chain terminator has been utilized in the manufacturing process, and to articles fabricated from such polycarbonate.
It is known to produce polycarbonate by the reaction of a dihydroxy compound, particularly a diphenol, with a carbonic acid derivative such as phosgene. A chain terminator is often added to the reaction mixture for the purpose of regulating the molecular weight of the polycarbonate product. When used, a chain terminator is typically included in the monomer mix which is initially charged to the reaction vessel, and the dihydroxy compound and chain terminator are consequently both present in the reaction mixture when it is contacted with the carbonic acid derivative.
In such a method of producing polycarbonate, molecules of varying degrees of polymerization are obtained. The chain terminator is useful for the purpose of attempting to confine a large percentage of the molecules to a preselected molecular weight range. However, even when a terminator is used according to the practice in the art as described above, not all molecules conform to the preselected weight range, and frequently some molecules of extremely low molecular weight (carbonate byproducts) are nevertheless produced.
Carbonate byproducts, being the smallest of the molecules resulting from the production of polycarbonate, have the lowest respective boiling points and are therefore the most likely to exist in vapor phase. The presence of carbonate byproducts in vapor phase becomes a consideration when polycarbonate is, for example, being extruded, molded, or heated for any other purpose. Accordingly, it would be desirable to prevent or limit the formation of carbonate byproducts in polycarbonate, and obtain a polycarbonate product wherein the greatest possible percentage of the molecules fall within the preselected molecular weight range which it is the purpose of a chain terminator to achieve.
For the purpose of reducing the formation of carbonate byproducts in polycarbonate, U.S. Pat. No. 4,939,230 (which is incorporated herein in its entirety) discloses the general procedure of withholding addition of a chain terminator to a polycarbonate reaction mixture until reaction of the dihydroxy compound and carbonic acid derivative has proceeded to completion. However, additional process steps concerning the timing of admixture of base or a coupling catalyst with the reaction mixture have been found to be a valuable refinement of that general procedure.